The present invention relates to a spinning piezoelectric beam of a dual-axis angular rate sensor which is employed for attitude control of navigable vehicles such as aircraft. The invention also pertains to a method for adjusting the piezoelectric beam.
FIG. 1 illustrates a conventional spinning beam type, dual-axis angular rate sensor. The two end plates 11a and 11b of a cylindrical case 11 have mounted thereon bearings 12 and 13, respectively, and a rotary shaft 21 of a spinning piezoelectric beam 20 passes through the bearings 12 and 13 and is held therebetween in a manner to be rotatable about the Z axis. Two parallel beam-shaped piezoelectric sensors 22a and 22b are affixed, by support washers 24, to the rotary shaft 21 at right angles thereto and symmetrically with respect thereto. The piezoelectric sensors 22a and 22b are produced, for example, by forming electrodes 25a and 25b on both sides of bimorph type piezoelectric crystal beams, and they are held perpendicular to the rotary shaft 21. Mounted on both sides of the piezoelectric sensors 22a and 22b at their free end portions are weights 23a and 23b for increasing the angular rate detecting sensitivity. The piezoelectric spinning beam 20 is driven at high speed by a motor comprising a stator 14 fixed in the case 11 and a rotor 15 fixedly mounted on the rotary shaft 21. The outputs of the piezoelectric sensors 22a and 22b, provided at the electrodes 25a and 25b, are led out via leads (not shown) extending through the rotary shaft 21, slip rings 24a, 24b and 24c, brushes 16a, 16b and 16c, and preamplifiers 31a and 31b.
Now, the X and Y axes which lie in a plane containing the piezoelectric sensors 22a and 22b and perpendicular to the Z axis and perpendicularly intersect each other are defined as shown in FIG. 2A. When the case 11 of the angular rate sensor is rotated about the X axis at an angular rate .OMEGA.x (shown as a vector indicated by the arrow in the X-axis direction), Coriolis force acts on the piezoelectric sensors 22a and 22b spinning about the Z axis at an angular rate .OMEGA.z, by which the piezoelectric sensors 22a and 22b bend in opposite directions at opposite sides with respect to the Y axis as depicted in FIG. 2A. As a result of this, the piezoelectric sensors 22a and 22b yield sine-wave voltage signals x.sub.a and x.sub.b which have an amplitude proposectional to the applied angular rate .OMEGA.x and are displaced 180.degree. apart in phase as indicated by the solid line and the broken line in FIGS. 2B. Also when the case 11 of the angular rate sensor is rotated about the Y axis at an angular rate .OMEGA.y, the piezoelectric sensors 22a and 22b similarly create sine-wave voltage signals y.sub.a and y.sub.b which have an amplitude proportional to the angular rate .OMEGA.y and are displaced 180.degree. apart in phase. The signals y.sub.a and y.sub.b are phased 90.degree. apart from the signals xa and x.sub.b, respectively. On the other hand, when a vibrational acceleration .alpha. acts on the piezoelectric sensors 22a and 22b in the Z-axis direction, the sensors 22a and 22b vibrate in the same phase as shown in FIG. 3A, yielding voltage signals z.sub.a and z.sub.b which are of the same magnitude proportional to the acceleration .alpha. and of the same sign as depicted in FIG. 3B. Even if acceleration is applied in either of the X- and Y-axis directions, the piezoelectric sensors 22a and 22b will not yield voltage. Accordingly, when an angular rate vector and an acceleration vector are applied to the case of the angular rate sensor in given directions, if the difference between output voltage signals from the piezoelectric sensors 22a and 22b is produced by an electrical circuit, the signal component arising from the acceleration in the Z-axis direction is removed but a voltage signal which is the sum of vectors of an X-axis direction angular rate component (x.sub.a -x.sub.b)=x and a Y-axis direction angular rate component (y.sub.a -y.sub.b)=y is obtained. Since the signals x and y are phased 90.degree. apart, the X- and Y-axis angular rate vector components x and y can be separated by synchronous detection using sine- and cosine-wave reference signals.
In practice, however, it is difficult to affix the piezoelectric sensors 22a and 22b to the rotary shaft 21 accurately at right angles thereto and in a correct attitude, because of limitations on machining and assembling accuracy of parts. Moreover, where the composite centers of gravity of the weights 23a and 23b provided at the free ends of the piezoelectric sensors 22a and 22b do not lie on the center axes thereof, acceleration perpendicular to the rotary shaft will bend the piezoelectric sensors, and if their pendulum axes are not parallel to each other, their bending differs in magnitude, introducing an error in the difference between the output signals of the two piezoelectric sensors. In the output difference signal of the piezoelectric sensors 22a and 22b an error component proportional to linear acceleration remains unremoved. To eliminate this error signal, the prior art employs an arrangement in which another piezoelectric sensor is affixed to the rotary shaft for sensing linear acceleration perpendicular thereto and its output signal is added to that of the angular rate sensing piezoelectric sensor to correct the error signal (for example, Japanese Patent Application Laid Open No. 120914/86 which corresponds to U.S. Ser. No. 672,560 filed on Nov. 19, 1984). Thus, the conventional angular rate sensor calls for the piezoelectric sensor for correction use and is adapted to permit adjustment of the angle at which the piezoelectric sensor is affixed to the rotary shaft; hence, the prior art is inevitably complex in structure.
Furthermore, in the conventional angular rate sensor the electrodes of the piezoelectric sensors 22a and 22b each extend to the ends of support washers by which the sensor is affixed to the rotary shaft, and consequently, the outputs of the sensors 22a and 22b are affected by the stresses supporting them. For example, a preload on each of the bearings 12 and 13 due to their misalignment differs with angular positions and causes a difference between the support stresses which act on the two piezoelectric sensors 22a and 22b through the rotary shaft 21, and the stress difference varies with the rotational stress. This introduces a difference between the outputs of the piezoelectric sensors 22a and 22b and the output difference varies with the rotational frequency, providing an error in the angular rate signal.